Blanket overheat protector



1959 H. w. WALLACE- 2,914,645

BLANKET OVERHEAT PROTECTOR Filed March 6, 1958 [)7 van t or: Henag WhG/Awe;

United States Patent BLANKET OVERHEAT PROTECTOR Henry W. Wallace, GreensFarms, Conn., assignor to General Electric Company, a corporation of NewYork Application March 6, 1958, Serial No. 719,585

10 Claims. (Cl. 219-20) This invention relates to electric heatingdevices and in particular to improved overheat protection means for suchdevices.

One object of this invention is to'provide an electrical heater andseries connected control system to provide continuous overtemperatureprotection throughout the length of the heater.

Another object of this invention is to provide a simplified electricalheater and overtemperature protective system, particularly suited forelectrically heated bedcovers and the like.

A further object of this invention is to provide an improved electricalheater and control system such as for electrically heated bedcovers,operable on either alternating or'unidirectional power.

Still another object of this invention is to provide an electric heatingelement and overtemperature protective system in which a thermosensitivelayer has a substantially uniform voltage applied across it at any pointalong the length of the heater.

In carrying out the objects of my invention in one form thereof, 1utilize .two heater wires or resistors arranged physically in parallelwith a layer or film of temperaturesensitive material having a negativetemperature co-efiicient of impedance between the two separate heaterwires. The heater wires are electrically connected in series in a mannerresulting in application of a substantially uniform proportion of totalline voltage between the heating wires at any point along the layer oftemperature-sensitive material. A series connected circuit controllingdevice opens the entire circuit responsive to the increased current flowwhen said temperature-sensitive layer conducts significant current.

Other objects and advantages of my invention will become apparent as thefollowing description proceeds, and the features of novelty whichcharacterize my invention will be pointed out with particularity in theclaims annexed to, and, forming part of this specification.

In the drawing:

Fig. 1 is a schematic wiring diagram of a control circuit utilizing myinvention; and

Fig. 2 is a perspective view, partially in section, of a portion of myheaterconductor. I 7

Referring now to the drawing, in Fig. 1, I show a wiring diagram of anelectrical heater and overtemperature pro- .tective system such as maybe used for an electrically heated bedcover or the ilke. Power issupplied to my device by means of plug 1 which can be connected to anysuitable alternating or direct current power source. The

heating element 2 consists of two electrical resistors 3 and 4,separated by a layer of material 5 having a pronounced negativetemperature co-efiicient of impedance.

The; heating resistors 3 and 4, while being arranged physically inparallel are connected in electrical series in such a manner thatthereis one-half line voltage between any point on one resistor and thedirectly adjacent point of the other resistor throughout the length ofthe heating element 2, As shown, this is accomplished by connect- 2 ingopposite ends of the resistors together at the mid point of the seriescircuit. The circuit has a series control device 6 comprising an Onswitch 7, normally biased open, a latching member 8 for maintaining On"switch 7 closed, a manual Off button 9 and a low impedance automaticrelease device 10. While I have shown my circuit using an over-currentcircuit breaker, it should be understood that any suitable currentsensitive circuit opening device may be used. As applied to an automaticelectrically heated bedcover, I also include in my circuit an ambienttemperature responsive device 11 for controlling the power input to theheating element 2 in response to room temperature, in the mannerdisclosed by Kearsley Patent 2,195,958, assigned to the General ElectricCompany as is the instant application.

In Fig. 2, I show by way of example a portion of my heating element,partially in section. One of the resistors, for example, resistor 3, iswound or spiralled on a flexible insulating core. Typically, core 12 isof glass or other electrical insulating fibers. Resistor 4 is appliedphysically in parallel with resistor 3. However, for successfuloperation of the circuit shown schematically in Fig. 1, it is necessaryto provide some means for physical separation of resistors 3 and 4. As atypical example of one means for providing such physical separation, Ihave shown in Fig. 2 a wrap offlexible absorbent material 5 over thespiralled resistor 3. It will be understood, of course, that theabsorbent material could be applied directly to resistor 3 prior to itsapplication on core 12; or, similarly, a wrap of suitable absorbentmaterial could be applied to resistor 4 prior to its application inphysical parallel with resistor 3. Other means of providing positivephysical separation of conductors 3 and 4 in a flexible electrical cableassembly will be obvious to those skilled in the art. I

To provide the desired temperature-sensitive characteristic, theabsorbent material 5 is suitably impregnated with a compound whichexhibits a pronounced change in resistance at a given criticaltemperature. There are a number of materials which exist in acrystalline or powdered form at normal room temperature. While in such acrystalline or powdered, form, these materials are essentiallyelectrical insulators. These same materials at a critical elevatedtemperature become conductors of significant electrical currents. WhileI do not wish to be limited by the theory explaining this change inelectrical impedance, it appears that the marked change from aninsulator to a conductor occurs on a change in state of the material,for example, from a solid or crystalline form to a liquid form. Otherdesirable characteristics of the temperature-sensitive material are thatit remains chemically stable at the expected opera-ting temperatures,and that it is relatively inactive with respect to metals and othernearby materials throughout the expected operating range.

As a typical example, one of the materials which has proved useful inthe practice of this invention is oxalic acid. After the resistors 3 and4 and the absorbent material 5 have been applied in a cable form asdiscussed above, the assembly is passed through a molten bath of oxalicacid. A substantial quantity of the oxalic acid is absorbed into thematerial 5. When this assembly cools, it is true that the oxalic acidcrystalizes; but the crystals are of microscopic size and are physicallysupported within the porous or absorbent material 5. Thus, if at anytime thereafter the temperature of resistor 3 or 4, or both, is elevatedto a temperature above the melting point of the oxalic acid, aconductive path is provided through the impregnated absorbent material5, to provide an increase in current flow through current sensitivedevice 6. Oxalic acid has been found particularly suitable with respectto an electrically heated bedcover, since its melting temperature is inthe order of C., this temperature being Well below the scorchingtemperature of fabrics nor- 'mally used for electrically heatedbedcovers. I do not wish to be limited, however, only to oxalic acid,since a numberof other materials exhibit a similar eflect. For example,acetamide may also be used, this material having a melting point ofapproximately 81 C., upon which the material changes from an insulatingsolid to a conducting electrolyte. A number of other materials exhibitsimilar characteristics, with change from an insulating solid to aconducting electrolyte at a critical temperature. Thus, an electricheating device can be provided with a control system to limit operationto a given maximum temperature by proper selection of the material. Ingeneral, the critical temperature of the temperature-sensitive materialappears to be approximately the melting point.

After assembly of resistors 3 and 4 with core 12 and the impregnatedabsorbent material 5, the cable is encased within an extruded or closelyfitting insulating jacket 13. A number of materials are suitable forjacket 13 such as polyvinyl chloride. Jacket or sheath 13 should notonly insulate electrical resistors 3 and 4, but also should seal theassembly to prevent any loss of the thermosensitive compound. Also, thethermosensitive compound may be soluble in water or other solvent usedfor cleaning purposes in an electric bedcover application. Thus, theouter jacket 13 should be resistant to such solvents. It is to be notedparticularly that the entire assembly retains a flexibility of the sameorder as the fabric which would comprise an electrically heatedbedcover. The crystalline or powder-like temperature-sensitive compoundis confined in and carried by the absorbent material which separatesphysically the two electrical heaters.

Returning to the operation of my circuit, it can be seen in Fig. 1 thatthe circuit is energized by-depressing On button 14 which closes Onswitch 7. When On switch 7 is closed, latch 8, being biased to the rightas shown in Fig. l, maintains switch 7 closed. When an overheat ortemperature rise over the critical temperature occurs at any pointwithin heating element 2, significant current flows at that pointbetween the two heater conductors 3 and 4, thereby causing a sharp risein total current in the control system. This increase in currentenergizes device attracting latch 8 downward and away from switch 7,allowing switch 7 to open. To disconnect the circuit manually,depression of CE button 9 also moves latch 8 downward and away fromswitch 7, allowing switch 7 to open.

While I have shown and described a specific embodiment of my invention,I do not desire my invention to be limited to the particularconstruction shown and described, and I intend by the appended claims tocover all modifications coming within the true spirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electrical heater and control system therefor comprising: at leasttwo resistor sections physically arranged in closely spaced parallelrelationship; a mass of material maintaining said resistors in saidparallel relationship and characterized by flexibility and a negativetemperature co-eflicient of impedance; means electrically connectingsaid resistors in series to an electrical power source so that asubstantially uniform proportion of line voltage appears across anypoint of said mass of material along said resistors; and currentsensitive relay means in series with said resistors adapted to open thecircuit from the electrical power source upon occurrence of appreciableelectrical conduction through said mass of material.

2. An electrical heater and control system therefor comprising: tworesistors physically arranged in closely spaced parallel relationship; amass of material maintaining said resistors in said parallelrelationship and characterized by a negative temperature co-efiicient ofimpedance and flexibility; means electrically connecting said resistorsin series to an electrical power source so that onehalf the line voltageappears across any point of said mass of material along said resistors;and a current sensitive relay in series with said resistors adapted toopen the circuit from the electrical power source upon occurrence ofappreciable electrical conduction through said mass of material.

3. An electrical heater and control system therefor comprising: tworesistor sections physically arranged in closely spaced parallelrelationship; a mass of material maintaining said resistors in saidparallel relationship and characterized by flexibility and a negativetemperature co-efiicient of impedance; said mass of material and saidresistors being extremely flexible; means electrically connecting saidresistors in series to an electrical power source so that asubstantially uniform proportion of line voltage appears across anypoint of said mass of material along said resistors; and currentsensitive relay means in series with said resistors adapted to open thecircuit from the electrical power source upon occurrence of appreciableelectrical conduction through said mass of material.

4. In an electrically heated bedcover, two resistor sections physicallyarranged in closely spaced parallel relationship; a mass of materialmaintaining said resistors in said parallel relationship andcharacterized by flexibility and a negative temperature co-efficient ofimpedance; means electrically connecting said resistors in series to anelectrical power source so that one-half line voltage appears across anypoint of said mass of material along said resistors: and a currentsensitive relay in series with said resistors adapted to open thecircuit from the electrical power source upon occurrence of appreciableelectrical conduction through said mass of material.

5. In an electrically heated bedcover, at least two resistor sectionsphysically arranged in closely spaced parallel relationship; a mass ofmaterial maintaining said resistors in said parallel relationship andcharacterized by a negative temperature co-efficient of impedance; meanselectrically connecting said resistors in series to an electrical powersource so that a substantially uniform proportion of line voltageappears across any point of said mass of material along said resistors;and current sensitive relay means in series with said resistors adaptedto open the circuit from the electrical power source upon occurrence ofappreciable electrical conduction through said mass of material.

6. A temperature controlled flexible electric heater adapted for usewith an electrically heated bedcover or the like comprising: first andsecond heater sections of approximately equal electrical resistance;absorbent means maintaining said heater sections in parallel physicalrelationship; a temperature-sensitive impregnating material in saidabsorbent means which is essentially an insulator at normal operatingtemperatures and a conductor of significant current at temperaturesabove a critical temperature; electrical conducting means connectingsaid heater sections in electrical series with one end of the firstheater section connected to the opposite end of the second heatersection, whereby approximately one-half total heater voltage appearsacross said impregnated absorbent means at any point between said heatersections; and current sensitive control means in an electrical circuitwith said heater sections adapted to open the electrical circuit to saidheater sections upon conduction of appreciable current through saidtemperaturesensitive impregnating material.

7. The combination of claim 6 wherein said temperature-sensitivematerial is oxalic acid.

8. The combination of claim 6 wherein said temperature-sensitivematerial is acetamide.

9. The combination of claim 6 wherein said temperature-sensitivematerial changes from a solid insulator to a conductive electrolyte atsaid critical temperature.

10. A temperature controlled heater comprising: first and second heatersections; means maintaining said heater sections in parallel physicalrelationship; a temperaturesensitive compound in intimate physicalcontact with said heater sections along the length thereof; saidtemperaturesensitive compound being an electrical insulator at normaloperating temperatures and becoming a conductive electrolyte upon achange in state to a liquid condition at temperature above a criticaltemperature; electrical conducting means connecting said heater sectionsin an electrical series with one end of the first heater sectionconnected to the opposite end of the second heater section, whereby apredetermined proportion of total heater voltage appears across saidtemperature-sensitive compound at any point between said heatersections; an outer enclosing impervious sheath for said heater sections;and current sensitive control means in an electrical circuit ReferencesCited in the file of this patent UNITED, STATES PATENTS 2,413,125Walbridge Dec. 24, 1946 10 2,487,526 Dahm et al. Nov. 8, 1949 2,768,274Estes Oct. 23, 1956 FOREIGN PATENTS 577,128 Great Britain May 6, 1946

